It depends on how often you trim plants, replant and live stock, I clean mine monthly~~max 2 months.... with 50%~70% water change weekly. Tho' you feel like its not need to clean, but actually there is...for flow rate and if you are using inline reactor its get clog.

@Ryu1~~No wonder why you have those nasty Algae in your plants....Clean often. 1 year is a way way to long to wait to clean..